Guides
Plain-language explanations of the math behind our calculators, with worked examples you can follow by hand.
- How mortgage payments are calculatedThe amortization formula explained from first principles, with a month-by-month example and the effect of rate, term and extra payments.7 min read
- Compound interest explainedInterest on interest, step by step: the formula, the effect of compounding frequency, the Rule of 72 and regular contributions.8 min read
- How to calculate percentagesPercent of a number, what percent one number is of another, percentage change and working backwards — with the traps that catch people out.6 min read
- What BMI tells you — and what it doesn’tThe history of BMI, why it is a screening number rather than a diagnosis, and how waist measurements add what it misses.6 min read
- How to count the days between two datesInclusive or exclusive, leap years, month lengths and business days — the rules behind every date difference, with examples.5 min read